Four months after a bi-polar schizophrenic from Maple Shade, NJ confessed to the murder of Etan Patz, investigators have zero additional evidence to substantiate a case against him, The Post has learned.

“There’s just nothing else on this guy,” said a source — one of several DA and NYPD investigators connected with the case who spoke of their frustration on condition of anonymity.

Investigators have scoured the walls and floors of a former SoHo bodega where Pedro Hernandez insists that in 1979 he strangled the 6-year-old boy and stuffed the body into a heavy cardboard banana carton for garbage men to haul away.

They’ve also spent countless hours in New Jersey, interviewing everyone Hernandez has ever known, seizing any computer he might have used and examining every nook of his residence — most promisingly finding a box hidden in the ceiling rafters of his attic bedroom.

The box held a pair of boys’ white Fruit of the Loom briefs, a yellow Matchbox-style car and a pair of blue children’s shorts — ultimately yielding no DNA, and no link whatsoever to Patz, sources say.

Investigators say they are essentially where they started in May — in possession of Hernandez’s confession plus several hearsay echoes of that confession by people close to Hernandez, who say that over the years he has “confessed” to them as well.

Hernandez’s own confession — actually a total of four statements — is universally described by people who have viewed it as lengthy and compelling.

“I know why you’re here,” he tells investigators who come for him in New Jersey. “I’ve been waiting for this for years,” he says. In his subsequent statements, he appears sane, measured and remorseful, sources say.

The details he provides, at times tearfully, are graphically detailed — but without a corpse or other forensics, utterly unable to be substantiated.

“He either did it, or he really believes he did it,” one DA source told The Post.

“It would have been crazy not to proceed, given this confession,” explained another DA source.

Still — and especially within the DA’s office — frustrations run high among many who believe that with all his serious mental issues, Hernandez should have merely been watched and investigated, not arrested and paraded before the press based on his own words alone.

Police Commissioner Ray Kelly brought tremendous pressure on the DA’s office to go forward with the high-profile bust, they complain — and solely to rub the case in the nose of the feds, who had irked Kelly days earlier by seizing control of the investigation, digging up another SoHo building’s basement in an ultimately fruitless search for Etan’s corpse.

“Politics,” investigators complain of the decisions to proceed from arrest through indictment and, eventually, trial. “It’s political.” An NYPD spokesman declined to comment on accusations that Kelly pressured the DA’s office.

The DA’s office has insisted that in prosecuting Hernandez, it is not yielding to NYPD pressure.

For now, prosecutors have still not sought a grand jury indictment. The indictment deadline, already postponed once, is currently Oct. 1, but that date too will be pushed back to Nov. 15 to give the DA time to hear a presentation by Hernandez’s lawyer, Harvey Fishbein, according to a law enforcement source.

The expectation among investigators is that the confession — and the witnesses who say Hernandez has in the past emotionally admitted to killing a little boy in New York — will be enough to win an indictment.

But the evidence as it now stands is legally insufficient to win a conviction at trial, investigators have said.